In sleep labs, where patients are diagnosed for sleep disorders such as OSA (Obstructive Sleep Disorder), breath flow monitors are commonly used to recognize apnea events—periods where there is a loss of the patient's breath, as well as Hypopnea events—periods where there is a substantial reduction of the patient's tidal volume. These breath flow monitors often evaluate and display the patients breath flow characteristics using pressure or thermistor sensor technologies.
When using pressure sensor technologies, the patient is usually interfaced with a breath flow measurement cannula (nasal or oral/nasal) that is connected, at the instrument side, to a sensitive pressure sensor. Pressure changes detected are proportional to flow, and hence evaluation of the changing pressure felt along the cannula provides a breath flow pattern relative to the patient flow dynamics.
With thermistor technologies, an electronic line is used between the instrument and patient interface, and a thermistor is placed in proximity to the nose. The thermistor is sensitive to the flow of air passing across it, which creates slight changes in its temperature.
Despite the broad use of breath flow measurements, especially in sleep labs, usage of a flow meter alone to measure falls in tidal volume may be, in some situations, unreliable. First, a flow meter requires an oral-nasal cannula, since nasal alone would falsely define a hypopnic event when a patient would breath alternately between nose and mouth, and even when an oral nasal cannula is used, the strength of the flow pattern changes considerably when moving from nasal to oral breathing, which again can be picked up as a false hypopnic event. Second, movement of the cannula during the patient's sleep can also cause erroneous changes in the detected flow amplitude. Third, if the patient's mouth opens beyond a certain degree while asleep, the pressure created by the oral breathing is dispersed over the entire opening, thus decreasing the amount of pressure picked us by the flow meter (although generally, the larger the oral breath collection inlet is, the smaller the problem). Such occasions are very common in sleep labs, since patients who arrive at the sleep lab seeking diagnosis of a suspected Obstructive Sleep Apnea (OSA), tend to experience snoring as a symptom; snoring, in turn, usually occurs with an open mouth, and therefore may indirectly trigger a false hypopnic event.
In contrast to breath flow measurement, the concentration of carbon dioxide (CO2) collected using an appropriate nasal or oral/nasal cannula and transported to a capnograph, is usually far less influenced from the position of the cannula or whether it is collected from the nose or the mouth. CO2 level measurement, or “Capnography”, is often defined as the measurement of the level of CO2 in exhaled and/or inhaled breath. Since infrared light was found to be absorbed particularly well by CO2, capnographs usually measure infrared absorption in the breath gasses, which indicates the level of CO2 in these gasses. Other measurement technologies exist as well.
The information obtained from a capnographic measurement is sometimes presented as a series of waveforms, representing the partial pressure of CO2 in the patient's exhaled breath as a function of time.
Clinicians commonly use capnography in order to assess a patient's ventilatory status. Respiratory arrest and shunt may be speedily diagnosed, and a whole range of other respiratory problems and conditions may be determined by the capnographic measurement. Capnography is considered to be a prerequisite for safe intubation and general anesthesia, and for correct ventilation management.
Sleep apnea is a disorder that commonly affects more than 12 million people in the United States. It takes its name from the Greek word apnea, which means “without breath.” People with sleep apnea literally stop breathing repeatedly during their sleep, often for a minute or longer and as many as hundreds of times during a single night.
Sleep apnea can be caused by either complete obstruction of the airway (obstructive apnea) or partial obstruction (obstructive hypopnea—hypopnea is slow, shallow breathing), both of which can wake one up. There are three types of sleep apnea—obstructive, central, and mixed. Of these, obstructive sleep apnea (OSA) is the most common. OSA occurs in approximately 2 percent of women and 4 percent of men over the age of 35.
The exact cause of OSA remains unclear. The site of obstruction in most patients is the soft palate, extending to the region at the base of the tongue. There are no rigid structures, such as cartilage or bone, in this area to hold the airway open. During the day, muscles in the region keep the passage wide open. But as a person with OSA falls asleep, these muscles relax to a point where the airway collapses and becomes obstructed.
When the airway closes, breathing stops, and the sleeper awakens to open the airway. The arousal from sleep usually lasts only a few seconds, but brief arousals disrupt continuous sleep and prevent the person from reaching the deep stages of slumber, such as rapid eye movement (REM) sleep, which the body needs in order to rest and replenish its strength. Once normal breathing is restored, the person falls asleep only to repeat the cycle throughout the night.
Typically, the frequency of waking episodes is somewhere between 10 and 60. A person with severe OSA may have more than 100 waking episodes in a single night.
The primary risk factor for OSA is excessive weight gain. The accumulation of fat on the sides of the upper airway causes it to become narrow and predisposed to closure when the muscles relax. Age is another prominent risk factor. Loss of muscle mass is a common consequence of the aging process. If muscle mass decreases in the airway, it may be replaced with fat, leaving the airway narrow and soft. Men have a greater risk for OSA. Male hormones can cause structural changes in the upper airway.
Other predisposing factors associated with OSA include:
Anatomic abnormalities, such as a receding chin;
Enlarged tonsils and adenoids, the main causes of OSA in children;
Family history of OSA, although no genetic inheritance pattern has been proven;
Use of alcohol and sedative drugs, which relax the musculature in the surrounding upper airway;
Smoking, which can cause inflammation, swelling, and narrowing of the upper airway;
Hypothyroidism, acromegaly, amyloidosis, vocal cord paralysis, post-polio syndrome, neuromuscular disorders, Marfan's syndrome, and Down syndrome; and
Nasal congestion.
The foregoing examples of the related art and limitations related therewith are intended to be illustrative and not exclusive. Other limitations of the related art will become apparent to those of skill in the art upon a reading of the specification and a study of the figures.